AJ Preller is the most overrated GM in the league. For as much was made about the Padres rebuild, there is very little actual homegrown talent that they themselves developed. The vast majority of that Padres roster is built either through trades (Cronenworth, Pham, Grisham, Myers, Musgrove, Darvish, Snell) or free agency (Hosmer, Machado). Tatis is really the only "homegrown" guy, and I think it's safe to say he would've been a superstar anywhere. There's a legitimate question to be raised about their player development.
I think he's a fantastic amateur scout, but his decision making when it comes to major league transactions is also questionable at best. I cannot fathom how any major league GM gives Eric Hosmer $144M when there was such a consensus that he was a landmine.